of all inflows from outside the system for all food-chain-base compartments 

 (i.e., phytoplankton and organic materials), (4) assimilation coefficients for 

 each food of each predator, and (5) selectivity weighting factors for alternative 

 prey of each predator. If assimilation rates are not known and cannot be even 

 roughly estimated or if selectivity is not thought to be important, A^ j and 

 W^ a can be eliminated from the above equations. 



Calculation of initial feedbacks of unassimilated food to organic material 

 compartments and of mineralized nitrogen to the nitrogen compartment are included 

 in this iterative procedure. A computerized routine calculates inflows and 

 outflows of nitrogen at steady state in order to set the constants relating 

 nitrogen excretion rates to respiration rates. In this routine, the inflow of 

 nitrogen to each compartment is determined by multiplying the rate of feeding 

 on each food source at steady state by the nitrogen concentration of that food 

 source. The quantity of nitrogen used in growth and predation is determined by 

 multiplying growth and predation rates by body nitrogen concentration. The 

 remainder is apportioned between elimination in excrement (N) and elimination 

 in fecal material (NF). (Excrement is the elimination of metabolized body 

 wastes, usually in the form of urine. Fecal material consists of ingested 

 material that was not assimilated.) Constants relating nitrogen mineralization 

 rates to respiration rates are then calculated for use in the model. 



In ecosystem modeling, the usual method for calculating the flows of 

 nitrogen in excrement and fecal material is to make the nitrogen concentrations 

 in excrement (carbon-dioxide-equivalent sugar) and fecal material equal to the 

 nitrogen concentration of the organism. Calculations based on values from a 

 laboratory study by Darnell and Wissing (1975) indicate that nitrogen concentrations 

 in excrement or fecal material are not equal to the nitrogen concentration of 

 the organism or even to the weighted nitrogen concentration of its food sources. 

 Furthermore, if such relationships are assumed, nitrogen inflows will not equal 

 nitrogen outflows to each compartment when organic flows are in steady state, 

 if nitrogen concentrations of the various compartments differ. The method used 

 in quantification of this model should be more accurate than that ordinarily 

 used and should assure more realistic results. 



The necessary input information for the nitrogen initialization routine is 

 (1) body nitrogen concentrations for all compartments (except high-nitrogen 

 organic, which had to be calculated in the routine) and (2) ratios of nitrogen 

 released in excrement to nitrogen released in feces from the animal compartments. 

 Nitrogen concentration of the high-nitrogen organic compartment was a variable 

 of the model that depended upon the mix of zooplankton fecal pellets and fish 

 discards in the compartment. 



Computer programs for the top-down iterative flow-balancing procedure and 

 the model were written in Microsoft BASIC and were run on an Ithaca Intersystems 

 Z80-based microcomputer. The Euler method of numerical integration was used in 

 the simulations. The iteration interval for the simulation was 0.1 day. 



QUANTIFICATION 



The initial conditions, rate-coefficients, and other constants that were 

 required to run the model are shown in table 1. The values used for each of 

 these parameters and the calculations and references that were the bases for 



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